The invention relates to a noise eliminating circuit for a graphical input terminal and, more particularly, to a noise eliminating circuit suited to eliminate impulsive noises superimposed on coordinate signals, or the like.
A conventional graphical input terminal, which is described in U.S. Pat. No. 3,959,585, generates an electrical indication which is representative of the position of a stylus as it is hand-operated on an input surface. However, in this terminal, fine impulsive noise is generated due to irregular fluctuations in writing pressure on the input surface. These noises are superimposed on the electrical indication, thereby impairing an accurate detection of the position of the stylus on the surface.
In order to obviate such a disadvantage, another input terminal has been described to eliminate such noises. For this purpose, this terminal is equipped with an additional noise eliminating circuit, which includes a low-pass filter having a resistor and a capacitor. However, with this terminal, an accurate detection of the position of the stylus cannot be attained during a period of time immediately after a stylus-moving or a writing operation has started, as described in detail hereinafter.
One object of this invention is, therefore, to provide a noise eliminating circuit for a graphical input terminal which is free from above-mentioned disadvantages in the prior art terminals.
According to one aspect of the present invention, there is provided a noise eliminating circuit for a graphical input terminal having an input unit. The input unit generates an electrical indication corresponding to information represented on the input surface thereof by a hand-operated stylus and further generates a signal including information specifying the starting time of a writing operation on the surface. The terminal further comprises: low-pass filter means responsive to the indication; switch means for selecting as an output signal to the noise eliminating circuit either one of the indication or the output signal from the low-pass filter means; and control means responsive to the signal including the starting time information to control the switch means for selecting the indication for a predetermined duration of time after the starting time.